Raman scattering in a molecule occurs when light impinges upon the molecule and interacts with the electron cloud and the bonds of that molecule. For the spontaneous Raman effect, which is a form of light scattering, a photon excites the molecule from the ground state to a virtual energy state. When the molecule relaxes it emits a photon and it returns to a different rotational or vibrational state. The difference in energy between the original state and this new state leads to a shift in the emitted photon's frequency away from the excitation wavelength. The Raman effect, which is a light scattering phenomenon, should not be confused with absorption (as with fluorescence) where the molecule is excited to a discrete (not virtual) energy level.
If the final vibrational state of the molecule is more energetic than the initial state, then the emitted photon will be shifted to a lower frequency in order for the total energy of the system to remain balanced. This shift in frequency is designated as a Stokes shift. If the final vibrational state is less energetic than the initial state, then the emitted photon will be shifted to a higher frequency, and this is designated as an Anti-Stokes shift. Raman scattering is an example of inelastic scattering because of the energy transfer between the photons and the molecules during their interaction.
Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system, such as a molecule. It relies on inelastic scattering,also know as Raman scattering, of monochromatic light, for example from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system.
Raman spectroscopy is commonly used in chemistry, since vibrational information is often specific to the chemical bonds and symmetry of molecules. Therefore, the Raman spectrum of a molecule (that is, a measure of Raman scattering as a function of wavelength of light incident on the molecule) provides a fingerprint by which the molecule can be detected and identified.
Surface enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on a surface. The enhancement factor can be significant (for example, a factor on the order of 107 or more). This enhancement may increase the available signal strength for Raman spectroscopy, allowing for molecular detection and identification when only a small sample is available. In some cases, SERS enhancement allows for detection on the single molecule level.
Tip enhanced Raman scattering (TERS) refers to SERS type enhanced scatter effect for molecules adsorbed on a portion of a surface formed as a tip (typically having a size of about 1 nm to about 100 nm). In some cases TERS provides increased scattering enhancement due, for example, to localized electric field effects in the vicinity of the tip.